chore: add types to gitignore.ts

[HannahThor]

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Summary

Add types for gitignore.ts file

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[github-actions]

📊 Benchmark results

Comparing with 9e4b616

• Dependency count: 1,329 (no change)
• Package size: 310 MB (no change)
• Number of ts-expect-error directives: 997 ⬇️ 0.40% decrease vs. 9e4b616

[sarahetter]

Hi Hannah! This looks good to me, thank you! We've got some tests failing for external contributors that'll block me merging this until we work out what's going on there, but I'll keep an eye on it and merge when ready.